Flash memory devices may be broadly classified into two groups: One is a NOR-type flash memory device and the other is a NAND-type flash memory device. Compared with NAND-type flash memory device, a NOR-type flash memory device has better read data performance and supports execute-in-place (XIP) applications to execute programs directly. Therefore, a NOR-type flash memory is widely used in embedded systems to store program data and execute program codes.
However, the inventors have discovered that the threshold voltage of a NOR or NAND flash memory device undesirably increases with an increase in the integration density of the flash memory device due to the oxygen smiling effect, thereby adversely affecting the performance of the flash memory device.